1. Technical Field of the Invention
This invention relates to a spark plug for internal combustion engines used for automobiles, cogenerations, gas feeding pumps, etc. and a method for manufacturing the spark plug.
2. Description of the Related Art
Spark plugs are still used abundantly as an ignition means of a fuel-air mixture introduced into a combustion chamber of internal-combustion engines used for automobiles, etc.
As an example, a spark plug disclosed in Japanese Patent Application Laid-Open Publication No. 11-204233 is known.
The spark plug disclosed in the publication is shown in FIGS. 34 and 35. As shown in the figures, the spark plug 9 has a center electrode 94 and a ground electrode 95.
The ground electrode 95 is fixed to a mount fitting 92, while the ground electrode 95 has a projection part 951. The projection part 951 is made of a part of base materials of the ground electrode 95 where the part is facing the center electrode 94, and is projected toward the center electrode 94. A precious-metal-chip 953 is welded to an electric discharge surface 952 of the projection part 951.
However, there are the following problems in the above-mentioned spark plug 9.
That is, the precious-metal-chip 953 and the base material of the ground electrode 95 are joined only at a joint part 954. In another words, the precious-metal-chip 953 and the base material of the ground electrode 95 are welded only on the circumference of contacting surfaces of the projection part 951 and the precious-metal-chip 953.
For this reason, if heat stress acts on the joint part 954 by a thermal cycle, a crack or oxidization will take place in the joint part 95 fourthus the reliability in joint of the precious-metal-chip 953 and the base material of the ground electrode 95 will fall.
In addition, since the thermal conductivity of the joint part 954 is smaller than that of the precious-metal-chip 953, heat radiation of the precious-metal-chip 953 worsens, and there is a possibility that the temperature may rise thereof. Thereby, the precious-metal-chip 953 is worn and there is a possibility that demand voltage may become large.
Further, in order to lower demand voltage, it is necessary to make a spark discharge gap G formed between the center electrode 94 and the precious-metal-chip 953 small, but in this case, the amount of the precious metals used will increase, and there is a possibility that the cost of manufacturing the spark plug 9 may rise.